Power supply unit (computer) - AbsoluteAstronomy.com

A power supply unit converts mains AC

Mains electricity

Mains is the general-purpose alternating current electric power supply. In the US, electric power is referred to by several names including household power, household electricity, powerline, domestic power, wall power, line power, AC power, city power, street power, and grid power...

to low-voltage regulated DC power for the internal components of the computer. Modern personal computers universally use a switched-mode power supply

Switched-mode power supply

A switched-mode power supply is an electronic power supply that incorporates a switching regulator in order to be highly efficient in the conversion of electrical power...

. Some power supplies have a manual selector for input voltage, while others automatically adapt to the supply voltage.

Most modern desktop personal computer power supplies conform to the ATX form factor

ATX

ATX is a motherboard form factor specification developed by Intel in 1995 to improve on previous de facto standards like the AT form factor. It was the first big change in computer case, motherboard, and power supply design in many years, improving standardization and interchangeability of parts...

. ATX power supplies are turned on and off by a signal from the motherboard

Motherboard

In personal computers, a motherboard is the central printed circuit board in many modern computers and holds many of the crucial components of the system, providing connectors for other peripherals. The motherboard is sometimes alternatively known as the mainboard, system board, or, on Apple...

. They also provide a signal to the motherboard to indicate when the DC power lines are correct so that the computer is able to boot up. While an ATX power supply is connected to the mains supply it provides a 5 V stand-by (5VSB) line so that the standby functions on the computer and certain peripherals are powered. The most recent ATX PSU standard is version 2.31 of mid-2008.

Power rating and efficiency

Computer power supplies are rated based on their maximum output power. Power requirements for a modern desktop personal computer may range from 300 watts to more than 1000 watts for a file server or a computer with multiple processors. The power rating of a PC power is rated by the manufacturer; it is possible to overload one voltage from a power supply well below the total rating of the power supply. Simple, general purpose computers rarely require more than 300–350 watts maximum.

Appearance

Most desktop personal computer power supplies are a square metal box, and have a large bundle of wires emerging from one end. Opposite the wire bundle is the back face of the power supply, with an air vent and an IEC 60320 C14 connector to supply AC power. There may optionally be a power switch and/or a voltage selector switch. A label on one side of the box lists technical information about the power supply, including safety certifications and maximum output power. Common certification mark

Certification mark

A certification mark on a commercial product indicates five things:* The existence of a legal follow-up or product certification agreement between the manufacturer of a product and an organization with national accreditation for both testing and certification,* Legal evidence that the product was...

s for safety are the UL mark

Underwriters Laboratories

Underwriters Laboratories Inc. is an independent product safety certification organization. Established in 1894, the company has its headquarters in Northbrook, Illinois. UL develops standards and test procedures for products, materials, components, assemblies, tools and equipment, chiefly dealing...

, GS mark

Geprüfte Sicherheit

The Geprüfte Sicherheit or GS mark is a voluntary certification mark for technical equipment. It indicates that the equipment meets German and, if available, European safety requirements for such devices...

, TÜV

Technischer Überwachungsverein

TÜVs are German organizations that work to validate the safety of products of all kinds to protect humans and the environment against hazards. As independent consultants, they examine plants, motor vehicles, energy installations, devices and products TÜVs (short for Technischer...

, NEMKO

NEMKO

Norges Elektriske Materiellkontroll is a Norwegian private organization that supervises safety testing for electrical equipment manufacturing....

, SEMKO, DEMKO, FIMKO, CCC

China Compulsory Certificate

The China Compulsory Certificate mark, commonly known as CCC Mark, is a compulsory safety mark for many products sold on the Chinese market. It became effective on May 1, 2002...

, CSA

Canadian Standards Association

The Canadian Standards Association, also known as the CSA, is a not-for-profit Standards organization with the stated aim of developing standards for use in 57 different areas of specialisation...

, VDE

Verband der Elektrotechnik, Elektronik und Informationstechnik

The VDE Association for Electrical, Electronic and Information Technologies e.V. is one of Europe’s largest technical-scientific associations with 35,000 members, including 1,300 corporate and institutional members and 8,000 students.- Organization :...

, GOST R and BSMI. Common certificate marks for EMI/RFI

Electromagnetic interference

Electromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...

are the CE mark

CE mark

CE marking is a mandatory conformity mark for products placed on the market in the European Economic Area . With the CE marking on a product the manufacturer ensures that the product conforms with the essential requirements of the applicable EC directives...

, FCC

Federal Communications Commission

The Federal Communications Commission is an independent agency of the United States government, created, Congressional statute , and with the majority of its commissioners appointed by the current President. The FCC works towards six goals in the areas of broadband, competition, the spectrum, the...

and C-tick. The CE mark is required for power supplies sold in Europe and India.

A RoHS

Restriction of Hazardous Substances Directive

The Directive on the restriction of the use of certain hazardous substances in electrical and electronic equipment 2002/95/EC was adopted in February 2003 by the European Union. The RoHS directive took effect on 1 July 2006, and is required to be enforced and become law in each member state...

or 80 PLUS

80 PLUS

80 PLUS is an initiative to promote energy efficiency in computer power supply units . It certifies products that have more than 80% energy efficiency at 20%, 50% and 100% of rated load, and a power factor of 0.9 or greater at 100% load....

can also sometimes be seen.

Dimensions of an ATX power supply are 150 mm width, 86 mm height, and typically 140 mm depth, although the depth can vary from brand to brand.

Connectors

Typically, power supplies have the following connectors (all are Molex (USA) Inc Mini-Fit Jr, unless otherwise indicated):

PC Main power connector (usually called P1): This is the connector that goes to the motherboard
Motherboard
In personal computers, a motherboard is the central printed circuit board in many modern computers and holds many of the crucial components of the system, providing connectors for other peripherals. The motherboard is sometimes alternatively known as the mainboard, system board, or, on Apple...

to provide it with power. The connector has 20 or 24 pins. One of the pins belongs to the PS-ON wire (it is usually green). This connector is the largest of all the connectors. In older AT power supplies, this connector was split in two: P8 and P9. A power supply with a 24-pin connector can be used on a motherboard with a 20-pin connector. In cases where the motherboard has a 24-pin connector, some power supplies come with two connectors (one with 20-pin and other with 4-pin) which can be used together to form the 24-pin connector.
ATX12V 4-pin power connector (also called the P4 power connector). A second connector that goes to the motherboard (in addition to the main 24-pin connector) to supply dedicated power for the processor. For high-end motherboards and processors, more power is required, therefore EPS12V has an 8 pin connector.
4-pin Peripheral power connectors: These are the other, smaller connectors that go to the various disk drives of the computer. Most of them have four wires: two black, one red, and one yellow. Unlike the standard mains electrical wire color-coding
Domestic AC power plugs and sockets
AC power plugs and sockets are devices for removably connecting electrically operated devices to the power supply. Electrical plugs and sockets differ by country in rating, shape, size and type of connectors...

, each black wire is a ground
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....

, the red wire is +5 V, and the yellow wire is +12 V. In some cases these are also used to provide additional power to PCI cards such as FireWire 800 cards.
4-pin Molex (Japan) Ltd power connectors (usually called Mini-connector or "mini-Molex"): This is one of the smallest connectors that supplies the floppy drive with power. In some cases, it can be used as an auxiliary connector for AGP
Accelerated Graphics Port
The Accelerated Graphics Port is a high-speed point-to-point channel for attaching a video card to a computer's motherboard, primarily to assist in the acceleration of 3D computer graphics. Since 2004 AGP has been progressively phased out in favor of PCI Express...

video cards. Its cable configuration is similar to the Peripheral connector.
Auxiliary power connectors: There are several types of auxiliary connectors designed to provide additional power if it is needed.
Serial ATA power connectors: a 15-pin connector for components which use SATA power plugs. This connector supplies power at three different voltages: +3.3, +5, and +12 volts.
6-pin Most modern computer power supplies include 6-pin connectors which are generally used for PCI Express
PCI Express
PCI Express , officially abbreviated as PCIe, is a computer expansion card standard designed to replace the older PCI, PCI-X, and AGP bus standards...

graphics cards, but a newly introduced 8-pin connector should be seen on the latest model power supplies. Each PCI Express 6-pin connector can output a maximum of 75 W.
6+2 pin For the purpose of backwards compatibility, some connectors designed for use with high end PCI Express
PCI Express
PCI Express , officially abbreviated as PCIe, is a computer expansion card standard designed to replace the older PCI, PCI-X, and AGP bus standards...

graphics cards feature this kind of pin configuration. It allows either a 6-pin card or an 8-pin card to be connected by using two separate connection modules wired into the same sheath: one with 6 pins and another with 2 pins.
A IEC 60320 C14 connector with an appropriate C13 cord is used to attach the power supply to the local power grid.

AT vs. ATX

There are two basic differences between AT and ATX

ATX

power supplies: The connectors that provide power to the motherboard, and the soft switch. On older AT power supplies, the Power-on switch wire from the front of the computer is connected directly to the power supply.

On newer ATX power supplies, the power switch on the front of the computer goes to the motherboard over a connector labeled something like; PS ON, Power SW, SW Power, etc. This allows other hardware and/or software to turn the system on and off.

The motherboard controls the power supply through pin #14 of the 20 pin connector or #16 of the 24 pin connector on the motherboard. This pin carries 5V when the power supply is in standby. It can be grounded to turn the power supply on without having to turn on the rest of the components. This is useful for testing or to use the computer ATX power supply for other purposes.

AT stands for Advanced Technology when ATX means Advanced Technology eXtended.

Other form factors

The Thin Form Factor with 12 Volt connector (TFX12V) configuration has been optimized for small and low profile microATX and FlexATX system layouts. The long narrow profile of the power supply (shown in Figure 1) fits easily into low profile systems. The fan placement can be used to efficiently exhaust air from the processor and core area of the motherboard, making possible smaller, more efficient systems using common industry ingredients.

Laptops

Most portable computers have power supplies that provide 25 to 200 watt

Watt

The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...

s. In portable computers (such as laptop

Laptop

A laptop, also called a notebook, is a personal computer for mobile use. A laptop integrates most of the typical components of a desktop computer, including a display, a keyboard, a pointing device and speakers into a single unit...

s) there is usually an external power supply (sometimes referred to as a "power brick" due to its similarity, in size, shape and weight, to a real brick

Brick

A brick is a block of ceramic material used in masonry construction, usually laid using various kinds of mortar. It has been regarded as one of the longest lasting and strongest building materials used throughout history.-History:...

) which converts AC power to one DC voltage (most commonly 19 V), and further DC-DC conversion occurs within the laptop to supply the various DC voltages required by the other components of the portable computer.

Servers

Some web servers use a single-voltage 12 volt power supply.
All other voltages are generated by voltage regulator module

Voltage regulator module

A voltage regulator module or VRM, sometimes called PPM , is a buck converter that provides a microprocessor the appropriate supply voltage, converting +5 V or +12 V to a much lower voltage required by the CPU. Some are soldered to the motherboard while others are installed in an open slot...

s on the motherboard.

Energy efficiency

Computer power supplies are generally about 70–75% efficient. That means in order for a 75% efficient power supply to produce 75 W of DC output it would require 100 W of AC input and dissipate the remaining 25 W in heat. Higher-quality power supplies can be over 80% efficient; higher energy efficient

Energy conservation

Energy conservation refers to efforts made to reduce energy consumption. Energy conservation can be achieved through increased efficient energy use, in conjunction with decreased energy consumption and/or reduced consumption from conventional energy sources...

PSU's waste less energy in heat, and requires less airflow to cool, and as a result will be quieter. Google's server power supplies are more than 90% efficient. HP

Hewlett-Packard

Hewlett-Packard Company or HP is an American multinational information technology corporation headquartered in Palo Alto, California, USA that provides products, technologies, softwares, solutions and services to consumers, small- and medium-sized businesses and large enterprises, including...

's server power supplies have reached 94% efficiency. Standard PSUs sold for server workstations have around 90% efficiency, as of 2010.

It's important to match the capacity of a power supply to the power needs of the computer. The energy efficiency of power supplies drops significantly at low loads. Efficiency generally peaks at about 50–75% load. The curve varies from model to model (examples of how this curve looks can be seen on test reports of energy efficient models found on the 80 PLUS

80 PLUS

website). As a rule of thumb for standard power supplies it is usually appropriate to buy a supply such that the calculated typical consumption of one's computer is about 60% of the rated capacity of the supply provided that the calculated maximum consumption of the computer does not exceed the rated capacity of the supply. Note that advice on overall power supply ratings often given by the manufacturer of single component, typically graphics cards, should be treated with great skepticism. These manufacturers want to minimize support issues due to under rating of the power supply specifications and advise customers to use a more powerful power supply to avoid these issues.

Various initiatives are underway to improve the efficiency of computer power supplies. Climate savers computing initiative promotes energy saving and reduction of greenhouse gas emissions by encouraging development and use of more efficient power supplies. 80 PLUS

80 PLUS

certifies power supplies that meet certain efficiency criteria, and encourages their use via financial incentives. On top of that the businesses end up using less electricity to cool the PSU and the computer's themselves and thus save an initially large sum(i.e. incentive + saved electricity = higher profit).

Facts

Life span is usually measured in mean time between failures (MTBF). Higher MTBF ratings are preferable for longer device life and reliability. Quality construction consisting of industrial grade electrical components and/or a larger or higher speed fan can help to contribute to a higher MTBF rating by keeping critical components cool, thus preventing the unit from overheating. Overheating is a major cause of PSU failure. MTBF value of 100,000 hours (about 11 years continuous operation) is not uncommon.
Power supplies may have passive or active power factor correction (PFC). Passive PFC is a simple way of increasing the power factor
Power factor
The power factor of an AC electric power system is defined as the ratio of the real power flowing to the load over the apparent power in the circuit, and is a dimensionless number between 0 and 1 . Real power is the capacity of the circuit for performing work in a particular time...

by putting a coil in series with the primary filter capacitor
Filter capacitor
Filter capacitors are any capacitors used for filtering. Filter capacitors are common in electrical and electronic work, and cover a number of applications, such as:* Glitch removal on Direct current power rails...

s. Active PFC is more complex and can achieve higher PF, up to 99%.
In computer power supplies that have more than one +12V power rail, it is preferable for stability reasons to spread the power load over the 12V rails evenly to help avoid overloading one of the rails on the power supply.
- Multiple 12V power supply rails are separately current limited as a safety feature; they are not generated separately. Despite widespread belief to the contrary, this separation has no effect on mutual interference between supply rails.
- The ATX12V 2.x and EPS12V power supply standards defer to the IEC 60950 standard, which requires that no more than 240 volt-amps
  Volt-ampere
  A volt-ampere is the unit used for the apparent power in an electrical circuit, equal to the product of root-mean-square voltage and RMS current. In direct current circuits, this product is equal to the real power in watts...
  
  be present between any two accessible points. Thus, each wire must be current-limited to no more than 20 A; typical supplies guarantee 18 A without triggering the current limit. Power supplies capable of delivering more than 18 A at 12 V connect wires in groups to two or more current sensors which will shut down the supply if excess current flows. Unlike a fuse
  Fuse (electrical)
  In electronics and electrical engineering, a fuse is a type of low resistance resistor that acts as a sacrificial device to provide overcurrent protection, of either the load or source circuit...
  
  or circuit breaker
  Circuit breaker
  A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow...
  
  , these limits reset as soon as the overload is removed.
- Because of the above standards, almost all high-power supplies claim to implement separate rails, however this claim is often false; many omit the necessary current-limit circuitry, both for cost reasons and because it is an irritation to customers. (The lack is sometimes advertised as a feature under names like "rail fusion" or "current sharing".)
When the computer is powered down but the power supply is still on, it can be started remotely via Wake-on-LAN
Wake-on-LAN
Wake-on-LAN is an Ethernet computer networking standard that allows a computer to be turned on or woken up by a network message....

and Wake-on-ring
Wake-on-ring
Wake-on-Ring , sometimes referred to as Wake-on-Modem , is a specification that allows supported computers and devices to "wake up" or turn on from a sleeping, hibernating or "soft off" state Wake-on-Ring (WOR), sometimes referred to as Wake-on-Modem (WOM), is a specification that allows supported...

or locally via Keyboard Power ON (KBPO) if the motherboard supports it.
Early PSUs used a conventional (heavy) step-down transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

, but most modern computer power supplies are a type of switched-mode power supply
Switched-mode power supply
A switched-mode power supply is an electronic power supply that incorporates a switching regulator in order to be highly efficient in the conversion of electrical power...

(SMPS) with a ferrite-cored
Ferrite core
A ferrite core is a structure on which the windings of electric transformers and other wound components such as inductors are formed. It is used for its properties of high magnetic permeability coupled with low electrical conductivity .There are two broad applications for ferrite cores which...

high frequency
High frequency
High frequency radio frequencies are between 3 and 30 MHz. Also known as the decameter band or decameter wave as the wavelengths range from one to ten decameters . Frequencies immediately below HF are denoted Medium-frequency , and the next higher frequencies are known as Very high frequency...

transformer.
Computer power supplies may have short circuit protection, overpower (overload) protection, overvoltage protection, undervoltage protection, overcurrent protection, and over temperature protection.
Some power supplies come with sleeved cables, which is aesthetically nicer, makes wiring easier and cleaner and have less detrimental effect on airflow.
Since supplies are self-certified, a manufacturer's claimed output may be double or more what is actually provided. Although a too-large power supply will have an extra margin of safety as far as not over-loading, a larger unit is often less efficient at lower loads (under 20% of its total capability) and therefore will waste more electricity than a more appropriately sized unit. Additionally, computer power supplies generally do not function properly if they are too lightly loaded. (less than about 15% of the total load.) Under no-load conditions they may shut down or malfunction.For this reason the no-load protection was introduced in some power supplies.
The most important factor for judging a PSUs suitability for certain graphics cards is the PSUs total 12V output, as it is that voltage on which modern graphics cards operate. If the total 12V output stated on the PSU is higher than the suggested minimum of the card, then that PSU can fully supply the card. It is however recommended that a PSU should not just cover the graphics cards' demands, as there are other components in the PC that depend on the 12 V output.
Power supplies can feature magnetic amplifier
Magnetic amplifier
The magnetic amplifier is an electromagnetic device for amplifying electrical signals. The magnetic amplifier was invented early in the 20th century, and was used as an alternative to vacuum tube amplifiers where robustness and high current capacity were required...

s or double-forward converter circuit design.

Wiring diagrams

Modular power supplies

A modular power supply is an approach to cabling which allows users to omit unused cables. Whereas a conventional design has numerous cables permanently connected to the power supply, a modular power supply provides connectors at the power supply end, allowing unused cables to be detached from the power supply, producing less clutter, a neater appearance and less interference with airflow. It also makes it possible to supply a wider variety of cables, providing different lengths of Serial ATA

Serial ATA

Serial ATA is a computer bus interface for connecting host bus adapters to mass storage devices such as hard disk drives and optical drives...

power connectors instead of Molex connector

Molex connector

Molex connector is the vernacular term for a two-piece pin and socket interconnection, most frequently disk drive connectors. Pioneered by Molex Connector Company, the two-piece design became an early electronic standard. Molex developed and patented the first examples of this connector style in...

s.

While modular cabling can help reduce case clutter, they have often been criticized for creating electrical resistance. Some third party websites that do power supply testing have confirmed that the quality of the connector, the age of the connector, the number of times it was inserted/removed, and various other variables such as dust can all raise resistance. However, this is somewhat inconsequential as the amount of this resistance in a good connector is small compared to the resistance generated by the length of the wire itself.

Power supply rail

A power supply rail or voltage rail refers to a single voltage provided by a power supply unit (PSU) relative to some understood ground

Ground (electricity)

In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....

. Although the term is generally used in electronic engineering

Electronic engineering

Electronics engineering, also referred to as electronic engineering, is an engineering discipline where non-linear and active electrical components such as electron tubes, and semiconductor devices, especially transistors, diodes and integrated circuits, are utilized to design electronic...

, many people, especially computer enthusiasts, encounter it in the context of personal computer power supplies.

Original IBM PC standard

The original IBM PC power supply unit (PSU) supplied two main voltages: +5 V

Volt

The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...

and +12 V. It supplied two other voltages, −5 V and −12 V, but with limited amounts of power.

Most of the standard silicon microchips

Integrated circuit

An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...

of the time operated on 5 V power. Of the 63.5 watt

Watt

s these PSUs could deliver, most of it was on this +5 V rail.

The +12 V supply was used primarily to operate motors. Fan motors, floppy disk

Floppy disk

A floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles...

drives and later, hard disk

Hard disk

A hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...

drives. As more peripherals were added, more power was delivered on the 12 V rail. However, since most of the power is consumed by chips, the 5 V rail still delivered most of the power.

The −12 V rail was used primarily to provide the negative supply voltage to the RS-232

RS-232

In telecommunications, RS-232 is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE and a DCE . It is commonly used in computer serial ports...

serial ports.

An additional wire referred to as Power Good is used to prevent digital circuitry operation during the initial microseconds of power supply turn-on, where output voltages and currents are rising but not yet sufficient or stable for proper device operation. Once the output power is ready to use, the Power Good signal tells the digital circuitry that it can begin to operate.

ATX standard

When Intel developed the ATX

ATX

standard power supply connector (published in 1995), microchips operating on 3.3 V were becoming more popular, beginning with the Intel 80486DX4

Intel 80486DX4

The IntelDX4 is a clock-tripled i486 microprocessor with 16 kB L1 cache. Intel named it DX4 as a consequence of litigation with AMD over trademarks...

microprocessor in 1994, and the ATX standard supplies three positive rails: +3.3 V, +5 V, and +12 V. Earlier computers which wished to operate on 3.3 V typically used a simple but inefficient linear regulator

Linear regulator

In electronics, a linear regulator is a voltage regulator based on an active device operating in its "linear region" or passive devices like Zener diodes operated in their breakdown region...

to generate it from the +5 V rail.

The ATX connector provides multiple wires and power connections for the 3.3 V supply, because it is most sensitive to voltage drop

Voltage drop

Voltage drop is the reduction in voltage in the passive elements of an electrical circuit. Voltage drops across conductors, contacts, connectors and source internal resistances are undesired as they reduce the supplied voltage while voltage drops across loads and other electrical and electronic...

in the supply connections.

Another ATX addition was the +5sb rail for providing a small amount of standby power

Standby power

Standby power, also called vampire power, vampire draw, phantom load, or leaking electricity , refers to the electric power consumed by electronic and electrical appliances while they are switched off Standby power, also called vampire power, vampire draw, phantom load, or leaking electricity...

, even when the computer was nominally "off".

Increase in +12 V demand

As transistors become smaller on chips, it becomes preferable to operate them on lower supply voltages, and the lowest supply voltage is often desired by the densest chip, the central processing unit

Central processing unit

The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...

. In order to supply large amounts of low-voltage power to the Pentium and subsequent microprocessors, a special power supply, the voltage regulator module

Voltage regulator module

began to be included on motherboard

Motherboard

s.

Initially, this was supplied by the main +5 V supply, but as power demands increased, the high currents required to supply sufficient power became problematic. To reduce the power losses in the 5 V supply, with the introduction of the Pentium 4

Pentium 4

Pentium 4 was a line of single-core desktop and laptop central processing units , introduced by Intel on November 20, 2000 and shipped through August 8, 2008. They had a 7th-generation x86 microarchitecture, called NetBurst, which was the company's first all-new design since the introduction of the...

microprocessor, Intel changed the processor power supply to operate on +12 V, and added the separate P4 connector to the new ATX12V 1.0 standard to supply that power.

Modern high-powered graphics processing unit

Graphics processing unit

A graphics processing unit or GPU is a specialized circuit designed to rapidly manipulate and alter memory in such a way so as to accelerate the building of images in a frame buffer intended for output to a display...

s do the same thing, resulting in the vast majority of the power requirements of a modern personal computer

Personal computer

A personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end-user with no intervening computer operator...

being on the +12 V rail.

When high-powered GPUs were first introduced, typical ATX power supplies were "5 V-heavy", and could only supply 50–60% of their output in the form of 12 V power. Thus, GPU manufacturers, to ensure 200–250 watts of 12 V power (peak load, CPU+GPU), recommended power supplies of 500–600 W or higher.

More modern ATX power supplies can deliver almost all (typically 80–90%) of their total rated capacity in the form of +12 V power.

Because of this change, it is important to consider the +12 V supply capacity, rather than the overall power capacity, when using an older ATX power supply with a more recent computer.

Low-quality power supply manufacturers sometimes take advantage of this overspecification by assigning unrealistically high power supply ratings, knowing that very few customers fully understand power supply ratings.

+3.3 V and +5 V rails

As mentioned above, these supplies are rarely a limiting factor when selecting a power supply for a modern personal computer; generally any supply with a sufficient +12 V rating will have adequate capacity at lower voltages. However, a large quantity of hard drives or PCI cards will create a greater load on the +5 V rail. A linear regulator could be used to convert the +12 V rail into a +5 V rail for each hard drive if the +5 V rail is overloaded.

It is worth noting that most PSUs create their 3.3 V output by regulating down their 5 V rail. As such, 3.3 V and 5 V typically have a combined limit as well. For example, a 3.3 V rail may have a 10 A rating by itself (33 W), and the 5 V rail may have a 20 A rating (100 W) by itself, but the two together may only be able to output 110 W. In this case, loading the 3.3 V rail to maximum (33 W), would leave the 5 V rail only be able to output 77 W.

As all of the rails come from one transformer

Transformer

A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

and primary-side switching

Switched-mode power supply

A switched-mode power supply is an electronic power supply that incorporates a switching regulator in order to be highly efficient in the conversion of electrical power...

components, there is also an overall maximum power limit.

Multiple +12 V Rails

As power supply capacity increased, the ATX power supply standard was amended (beginning with version 2.0) to include:

This is a safety limit on the amount of power that may pass, in case of a fault, through any one wire. That much power can significantly overheat a wire, and would be more likely to melt the insulation and possibly start a fire.

Ideally, there would be one current limit per wire, but that would be prohibitively expensive. Since the limit is far larger than the reasonable current draw through a single wire, manufacturers typically group several wires together and apply the current limit to the entire group. Obviously, if the group is limited to , so is each wire in it. Typically, a power supply will guarantee at least at by having a current limit of , plus or minus 8%. Thus, it is guaranteed to supply at least , and guaranteed to cut off before .

These groups are the so-called "multiple power supply rails". They are not fully independent; they are all connected to a single high-current source inside the power supply, but have separate current limit circuitry. The current limit groups are documented so the user can avoid placing too many high-current loads in the same group.

This works in the same way, and for the same reason, as the many small circuit breakers in a circuit breaker panel

Distribution board

A distribution board is a component of an electricity supply system which divides an electrical power feed into subsidiary circuits, while providing a protective fuse or circuit breaker for each circuit, in a common enclosure...

as well as the main supply breaker. And just like typical domestic wiring, multiple outlets are connected to each circuit breaker for reasons of cost.

Originally, a power supply featuring "multiple +12 V rails" implied one able to deliver more than 20 A of +12 V power, and was seen as a good thing. However, people found the need to balance loads across many +12 V rails inconvenient. This problem was exacerbated by the fact that the assignment of connectors to rails is done at manufacturing time, and it is not always possible to move a given load to a different rail.

Rather than add more current limit circuits, many manufacturers have chosen to ignore the requirement and increase the current limits above 20 A per rail, or provide "single-rail" power supplies that omit the current limit circuitry. (In some cases, in violation of their own advertising claims to include it. For one example of many, see ) The requirement was deleted from version 2.3 (March 2007) of the ATX12V power supply specifications.

Operation of overcurrent protection

When a power supply has multiple-rail overcurrent protection, if any rail reaches that limit, the entire power supply will shut down. This is not associated with any overheating or increase in ripple voltage by the power supply as a whole, as might be caused by an overall overload. The only reliability penalty from operating a rail close to its current limit comes from the risk of triggering the shutdown.

External links

Computer power supply calculators

eXtreme Power Supply Calculator (frequently updated)
Snoop Power Supply Calculator
Computer Power Supply Calculator (outdated)
Journey Systems' online power supply calculator

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Power rating and efficiency

Appearance

Connectors

AT vs. ATX

Other form factors

Laptops

Servers

Energy efficiency

Facts

Wiring diagrams

Modular power supplies

Power supply rail

Original IBM PC standard

ATX standard

Increase in +12 V demand

+3.3 V and +5 V rails

Multiple +12 V Rails

Operation of overcurrent protection

See also

External links

Computer power supply calculators